The increasing number of various types of corrosive materials has accelerated the need to develop methods of treating the surface of sheets of steel to prevent their corrosion. A significant position in the wide range of these methods is occupied by thin galvanized sheet. The percentage of alloying metals plays a decisive part in relation to later processing.
The constant increase in the use of galvanized steel raises the question of how the material can be attached using known economical finishing processes. The special structure of the zinc coating produced for example by continuous galvanization entails no problems in shaping. The opposite, however, is true for welding. Of the welding processes most commonly used with steel sheet--spot, projection, and seam--spot welding is the most difficult because the electrode productivity can only be considered too low, especially for the economically preferable automatic welding.
"Electrode productivity" in this context means the number of welds of adequate quality that can be obtained with one electrode without reconditioning the contact surface of the electrode or the interface between the electrode and the material being welded. The quality of a spot weld can be determined by several methods, the most important of which are
the free shearing-tension test, PA1 the torsion test, PA1 the button test, and PA1 microscopic examination of the polished joint. PA1 alloy with zinc only slightly if at all, PA1 remain hard at high temperatures, and PA1 have a low electric resistance and satisfactory heat conductivity.
Extensive studies have accordingly been devoted to developing material-specific welding techniques and to either increasing electrode productivity or explaining why it is so low in spot-welding galvanized sheets.
Low electrode productivity derives from the tendency of zinc to alloy with the electrode's copper. The creation of an alloyed coating on the operating surface of the electrode changes the resistance of the spot-welding circuit. The resistance at the interface between the electrode and the sheet increases. The result is higher heat at the weld, which in turn accelerates the alloying of the electrode. The quality of the welds decreases rapidly as their number increases. It is accordingly necessary to make the electrodes out of a material that will not alloy with zinc. Tungsten, which has a high melting point, suggests itself for insertion into a copper electrode.
Attempts have been made to solve the problem in composite electrodes of the type known from German OS 1 565 318 by positioning the various components next to one another.
The object of both German OS 2 554 990 and German Patent 1 914 106 is to provide a longer-lived electrode by embedding a pin of heat-resistant material in its contact surface
German Patent 625 201 discloses an electrode with inserts of a difficult-to-melt metal, preferably in the form of a sheet embedded in the more readily melting metal with its grain more or less paralleling the direction of the electric current.
German AS 2 203 776 discloses electrodes with contact surfaces made of alloys with crystals that have longitudinally oriented separated phases.
U.S. Pat. No. 3,665,145 discloses electrodes with capped points and disks of high melting-point materials.
Professors Matting and Kruger of the Hanover Institute of Technology have thoroughly studied the lives of molybdenum-and-tungsten electrodes used for spot-welding thin galvanized sheet metal. The results are presented in the journal Bander Bleche Rohre 8 (1967), 5 & 6.
In spite of the range of this research, however, the results were not satisfactory. When the high melting-point inserts were very long and thick, they could not be water-cooled adequately enough to remove the heat. Again, a thick tungsten insert is a very poor conductor of electricity and impedes heat dissipation. Practical tests were accordingly mostly unsuccessful. It was impossible to reproduce welds with the same quality.
It was determined that the lack of reproducibility was of thermal origin. Cracks were discovered in the tungsten inserts, and the contact surface was perceptibly roughened. Small particles of tungsten were uniformly distributed over the whole area. Certain magnifications of the cracks revealed a medium-gray substance. It was accordingly easy to conclude that the zinc in the steel sheeting had become forced into the cracks during the welding process and, due to the intense heat, had diffused along the grain boundaries and into the tungsten. This situation has a certain explosive effect that allows the tungsten to escape. Simultaneously, however, it also increases the electrode's electric resistance, leading to an increase or readjustment of the welding current. The more powerful current, however, again increases the level of heat in the electrode. The diffused zinc creates an insulating layer and also prevents the transition of heat into the cooled copper. The buildup of heat, however, simultaneously promotes the creation of an alloy.
These tests were conducted on electrodes with tungsten or molybdenum inserts. The inserts were stripped from conventional rod material and were relatively long because they had to be reliably secured in the shaft of the electrode. The grain orientation, however, was along the current direction, parallel, that is, to the length of the electrode. The productivity of the electrodes with inserts of this type varied widely, and no definite reproducibility has been attained at the state of the art.
Specifically, a material for electrodes for spot-welding galvanized sheets must